html – How to make sure that a div does not overlap another div?

I have a div which is my vertical menu, next to it i want to put a div that fills all the remaining space that will be where to load my content, but what's happening, it's that & # 39; A div overlaps the other when I decrease the size browser:

I've posted a JSFiddle with the post pro css not getting so extended

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My JSFiddle: https://jsfiddle.net/cxbdpw8r/1/